Battery chemistry is extremely important in determining how long and under which load conditions battery powered systems can operate. Common primary battery chemistries include, but are not limited to, zinc manganese dioxide (i.e. alkaline), zinc carbon, primary lithium, biological, seawater, and thermal. Common secondary battery chemistries include, but are not limited to, nickel metal hydride, nickel cadmium, silver zinc, nickel zinc, lithium ion, and lead acid. For example, ubiquitous alkaline cells can provide a considerable amount of electrical energy if discharged very slowly over an extended period of time, i.e. at a low discharge rates. However, the available capacity of an alkaline cell is reduced significantly as the discharge rate increases. For example, a commercial alkaline D-size cell (e.g., a battery with zinc manganese dioxide chemistry) can supply approximately 20 ampere-hours (AH) at a constant current load of 50 milliamperes (mA). The capacity of the same alkaline D-size cell is reduced to only 4 AH at a load of 1000 mA (i.e. at higher current the cell capacity is reduced to only 20% of its capacity at low current). Accordingly, alkaline cell chemistry can efficiently supply energy to battery operated systems over long time periods only when the system to be powered requires relatively low and constant current. This condition is highly restrictive because many operational and proposed systems also include the requirement for intermittent high power demands.
Existing methods may drive the load directly from an alkaline battery or interpose a switching voltage regulator between the battery and the load in order to mitigate the varying voltage output of alkaline cell chemistry over loading and state-of-charge. When there is an intermittent, relatively high power load demand placed on the alkaline cell, the internal resistance of the alkaline cell increases with duration, causing the battery terminal voltage to drop to an unsatisfactory level, even when only a relatively small fraction of the gross (chemical) energy has actually been extracted from the battery. The output voltage fluctuation resulting from the relatively high internal resistance of the alkaline cell makes alkaline cells unattractive for sourcing high currents, even when a heavy load is only applied intermittently. In addition, when the load is driven directly from the battery, the voltage of the battery varies over a wide range as a function of the instantaneous load as well as the charge extracted previously. The open circuit voltage of an alkaline cell varies considerably over the discharge profile of the cell: starting out at 1.55 volts when the cell is new and dropping to approximately 0.8 volts at “end-of-life”. The variable voltage problem may be eliminated by interposing a switching voltage regulator between the battery and the load. However, after the open circuit voltage of the battery begins to drop and its output impedance increases as more charge is extracted over time, the voltage regulator demands more current in order to maintain a constant output voltage. This creates an unstable and inefficient condition where a large percentage of the energy extracted from the battery is internally dissipated by the internal resistance of the battery itself, leading to a substantial loss of net energy capacity due to voltage collapse of the partially depleted alkaline battery as charge is extracted.
With these characteristics, when an alkaline battery reaches a partial discharge state where it no longer can supply intermittent, high power load requirements, there still remains a significant amount of chemical energy (i.e. wasted energy) in the “dead” battery which cannot be utilized in practice. Thus, both of the existing methods require significant oversizing of the battery whenever a high power, intermittent load is to be driven. In other words, because a considerable percentage of the gross energy within the battery may be unavailable to drive the load due to the internal resistance of the battery, the net energy of the battery that remains available to drive the load is greatly reduced. To account for this, a system designer must incorporate a significant amount of excess battery capacity into the battery design, leading to an undesirable, physically larger and heavier battery, particularly when the peak load current which must be supplied is high and intermittent in nature.
A primary lithium cell (for example, a battery utilizing lithium thionyl chloride chemistry) has even higher energy density than an alkaline cell, can satisfy intermittent, high power load demands better than an alkaline cell, can provide relatively better voltage regulation than the alkaline cell, can provide reduced internal resistance compared with an alkaline cell, has better shelf life than the alkaline cell, and may be used at lower operating temperatures than the alkaline cell. However, because of the explosive nature of lithium cells, this chemistry cannot be used in certain applications because of the hazard they present.
A nickel metal hydride (NiMH) battery can satisfy intermittent, high power load demands better than either an alkaline or a primary lithium cell, can provide relatively better voltage regulation than either an alkaline cell or a lithium cell, and can provide relatively lower internal resistance than either an alkaline cell or a lithium cell. However, the NiMH battery has significantly lower energy density than either an alkaline cell or a lithium cell, and has a higher self-discharge rate than either an alkaline cell or a lithium cell. Accordingly, no single battery chemistry can be used to efficiently supply a battery operated system over long periods of time when the system requires modest average power, high total energy, high, intermittent peak power, and safe operation.
Accordingly, there is a need for hybrid battery power system that is configured to maximize the advantages of different battery chemistries given specific system requirements.